The present invention relates to a sublimation type thermosensitive image transfer recording medium.
Recently the demand for full color printers is increasing year by year. Representative recording methods for full color printers now available are the electrophotographic method, the ink-jet method, and the thermosensitive image transfer method. Of these methods, the thermosensitive image transfer method is most employed because of its advantages over the other methods that the maintenance is easy and the operation is noiseless.
In the thermosensitive image transfer recording method, a solidified color ink sheet and a receiving sheet are employed, and a color ink is transferred imagewise from the ink sheet to the receiving sheet by the thermal fusing of the ink or by the sublimation of the ink, with application of thermal energy by laser beams or a thermal head under the control by electric signals.
Thus, the thermosensitive image transfer recording method can be roughly classified into two types, a thermal fusing image transfer type and sublimation image transfer type. The sublimation image transfer type has the advantages over the thermal fusing type that halftone can be obtained without difficulty and image gradation can be controlled as desired because in this image transfer, a sublimable dye is in principle sublimated in the form of independent molecules in such an amount as to correspond to the amount of thermal energy applied thereto, for instance, through a thermal head. Therefore, it is considered that the sublimation image transfer type is most suitable for color printers.
The sublimation image transfer recording method, however, has the shortcoming that its running cost is high, because in this image transfer method, a yellow ink sheet, a magenta ink sheet, a cyan ink sheet and when necessary, a black ink sheet, are employed for obtaining a full-color image, with selective application of thermal energy to each ink sheet and discarded after the recording, even though large unused portions are remained in each ink sheet.
In order to eliminate this shortcoming, there are proposed (1) an equal speed mode in which an ink sheet and a receiving sheet are moved at the same speed for using the ink sheet in repetition and (2) an N-times use mode in which the running speed of the ink sheet is made smaller than that of the receiving sheet so that the overlappingly used portions of the ink sheet at the first use and the second use are shifted little by little.
In the sublimation type thermosensitive image transfer recording method, the sublimation and evaporation reaction is fundamentally a reaction of zero order. Therefore, in the equal speed mode, the ink sheet cannot be used multiple times for printing because the the printed image density significantly decreases as the number of the printing increases, particularly in high image density areas, even though a sufficient amount of a dye for multiple printing is contained in the ink layer of the ink sheet.
Furthermore, in the N-times use mode, the multiple printing performance is improved, but it has the risk that an ink layer and an image receiving layer become fused and improper running of the ink sheet takes place when the running speeds of both the ink layer and the image receiving layer are not made equal.